The semiconductor industry requires three-dimensional (“3D”) inspection and/or metrology process for silicon wafers. Such inspection can be used, for example, to test the through silicon via (“TSV”) and bump structure or the particle shape (e.g., size and height). Typical techniques for inspection or metrology include: (1) triangulation; (2) geometric shadow; (3) various confocal microscope techniques; and (4) white-light (or broadband light) interferometry. Triangulation and geometric shadow techniques are not precise enough for contemporary back-end of line (“BEOL”) applications. Confocal microscopy and interferometry techniques typically fail to meet throughput requirements.
White-light interferometry is known to be a high-resolution method for 3D inspection and metrology and has been used in the semiconductor industry. There are two types of such devices in the market: (1) scanning white-light interferometers (“SWI”) and (2) spectroscopic white-light interferometers. In SWI devices, either the sample (e.g., the wafer under inspection) or the inspection optics scan along a direction perpendicular to the wafer surface, such as the z-direction, for a distance. Multiple frames are taken at specific z-values to determine the height measurement for a specific x-y location on the wafer surface. Such SWI devices are robust, but are generally slow. Furthermore, this technique requires the sample to move to a field of view and be stabilized before a measurement is taken, which also limits speed. Likewise, throughputs for current spectroscopic white-light interferometers are also slow for the semiconductor industry.
An auto-focus mechanism is used for an optical probe (OP) in semiconductor inspection and metrology processes. In this technique, a chopper is used to test if the focal point is on, behind, or after the pre-set position. Light passes through the chopper to a bi-cell photodetector. The bi-cell photodetector and chopper are electronically connected with a lock-in amp. When the light is on-focus there is zero phase shift between the reference signal from the chopper and the signals from the two channels of the bi-cell photodetector. If the light is under-focus or over-focus, then the phase of a cell is shifted negative or positive, respectively, to a reference signal, and the phase of another cell is shifted in an opposite direction. With a 100× objective, this method can detect and servo-loop to control the focus better than 20 nm. However, the throughput of the chopper technique is slow.
Existing triangulation and geometric shadow techniques typically do not provide required accuracy and precision for 3D inspection when the target structure height shrinks below 10 μm. Confocal and interferometry methods often do not provide required throughput or are too expensive for 3D inspection. Therefore, what is needed is an inspection and metrology technique that can provide better accuracy, cost, and throughput.